Inorganic fillers are commonly incorporated into polymeric resin compositions in order to modify the properties of the resin. For example, the use of a filler generally increases the rigidity and stiffness of a resin. However, most inorganic fillers have a naturally hydrophilic surface which is therefore not easily wetted by polymeric resin compositions which are generally hydrophobic, or oleophilic. This problem is especially acute when the resin is in the form of a low-viscosity liquid because under these conditions it is difficult to dissipate sufficient energy in the composition by mechanical agitation to effect dispersion of the inorganic filler in the polymeric resin. A further disadvantage of conventional inorganic fillers is that they generally have associated with them a small, but significant, quantity of water. Some thermosetting resins, such as nylon produced by polymerising caprolactam, contain as catalysts Grignard reagents which react preferentially with the water associated with the filler so that their efficiency in the resin composition is impaired. As little as 100 ppm of water on the filler can completely stop polymerisation. Polyurethane resin compositions generally contain diisocyanates which again react with any water which is present in the composition to evolve carbon dioxide and cause foaming in the composition. Other resin compositions may contain amine catalysts or cobalt salts, both of which are sensitive to any water which may be present and tend to be adsorbed onto the surface of an untreated inorganic filler.